The preparation of nanoporous carbon membranes, previously called carbogenic molecular sieves (CMS) by those skilled in the art, which possess high mechanical strength, simple fabrication procedure and are readily assembled into modules is described by Foley et al. in U.S. patent application Ser. No. 08/671,698 (U.S. Pat. No.5,972,079) which is incorporated in its entirety by reference.
Membranes have gained considerable importance as an inexpensive, low energy alternative to distillation for separation of gases. In particular, sieving of molecules based purely on size differences has emerged as a mechanism for obtaining extremely high selectivities of a particular component.
Currently, inorganic membranes constitute the bulk of separation materials, mostly for their stability at high temperatures. Other potential candidates for use as membrane materials include zeolites, polymers, ceramics and Carbogenic Molecular Sieve materials (hereinafter sometimes referred to as xe2x80x9cCMSxe2x80x9d or xe2x80x9cCMS materialsxe2x80x9d). CMS materials have the advantage of being relatively inexpensive compared to zeolites, more temperature resistant than polymers and less brittle than ceramics. Numerous studies have shown that a relatively narrow pore size distribution of 4-6 xc3x85 can be obtained by controlled pyrolysis of CMS precursor materials. Thus, it would be advantageous to utilize CMS in the form of a membrane to perform molecular sieving.
CMS materials can be derived from natural sources such as wood and coconut shells, as well as synthetic polymer precursors. The basis for their sieving action arises from the complex microstructure, which has been described as consisting of a network of aromatic domains and amorphous carbon. Disclinations between the various domains result in predominantly slit-shaped pores than can exclude certain molecules on the basis of size and shape. However, unlike zeolites, which have a unique pore size, CMS typically has a distribution of pore sizes that can range from 3 to 10 xc3x85. One application of CMS is in the separation of nitrogen and oxygen using the pressure swing adsorption method. The kinetic diameters of the two molecules differ by a mere 0.2 xc3x85xe2x80x94but careful control of the pore size results in very high selectivities for oxygen. This example also demonstrates the difference between a CMS, which performs true molecular sieving, and an activated carbon, whose performance is based on the difference in the adsorption equilibrium of gases. As nitrogen is more strongly adsorbed on activated carbon than oxygen, it would be held back and would have to be desorbed when the separation was complete. In a CMS, however, the equilibrium uptakes of both gases are the samexe2x80x94hence, the time of sieving becomes important to obtain a high selectivity.
CMS materials have been synthesized using a variety of different polymeric precursors. The controlled deposition of pyrolyzed carbon to narrow pores in activated carbons and other supports has also been studied extensively. Established synthesis methods involve pyrolyzing the precursor at a high temperature in an inert gas flow. However, not all polymers can be utilized for CMS productionxe2x80x94this depends on whether they undergo cross-linking at high temperatures or not. The thermodynamically preferred structure for carbon at high temperatures is graphite. In the case of xe2x80x9cgraphitizingxe2x80x9d polymers like PVC, graphite-like layers are formed at around 1000xc2x0 C., which results in a considerable decrease in microporosity of the material. Hence, the resulting carbon is not suitable for gas separations. On the other hand, PAN, PVDC and PFA cross-link at high temperatures to stabilize the structure and prevent the formation of graphite layers. This xe2x80x9cnon-graphitizingxe2x80x9d character of the polymers is due to the presence of heteroatoms such as oxygen and nitrogen, as well as excess hydrogen. The pore sizes obtained are between 4-6 xc3x85, which make them ideal for use as molecular sieves.
CMS materials are globally amorphous and do not exhibit any long range order as evident in zeolites. X-ray diffraction studies, which can resolve features on a length scale of 25 xc3x85, do not reveal a distinct diffraction pattern for the microstructure. HRTEM studies of the structure combined with FFT analysis, can be used to determine the spacing between the graphite layers. The structure of CMS is thought to consist of a tangled network of ribbon-like aromatic regions. The evolution of the microstructure depends on the polymer precursor as well as the pyrolysis parameters of soak time and temperature. Investigations have shown that for most precursors, high temperature sintering leads to shrinkage of pores. There is, however, a collapse of the structure above a certain temperature, leading to a loss in the sieving property. A comprehensive review of CMS materials has been carried out by Foley (see Foley, H. C., Carbogenic Molecular Sieves: Synthesis, Properties and Applications; Microporous Materials, 1995;4; pp. 407-433).
Nanoporous membranesxe2x80x94porous membranes generally having a porosity below 1 nmxe2x80x94have attracted the attention of many researchers because of their potential for technological advances in gas separations and shape selective catalysis. (Saracco et al. 1994, infra.). Permeation experiments often constitute a significant contribution to the characterization of these membranes. As the dimensions of a pore approach that of the molecule, transport generally becomes extremely sensitive to the molecular dimensions of the probe gas and very high separation factors have been reported for ceramic, see Vercauteren, S., Keizer, K., Vansant, E. F., Lutyten, J., and R. Leysen, (1998), Porous Ceramic Membranes: Preparation, Transport Properties and Applications, J. of Porous Materials 5, 241, and zeolite, see Bai C, Jia M, Falconer J, et al. (1995), Preparation and Separation Properties Silicalite Composite Membranes, J. Membrane Science, 105, 79, and nanoporous carbon membranes of this type described herein.
There are two forms of CMS membranesxe2x80x94the unsupported xe2x80x9chollow fiberxe2x80x9d form, and the supported form. The hollow fiber membrane was developed by Koresh and Soffer (see Koresh, J. E. and A. Soffer, Molecular Sieve Carbon Permselective Membrane Part I. Presentation of a New Device for Gas Mixture Separation; Separation Science and Technology, 1983; 18(8); pp. 723-734) by pyrolysis of polyacrylonitrile (PAN) fibers. Despite their good sieving properties, the membranes lacked the requisite mechanical strength for use in various applications. A hollow fiber also cannot be converted easily into a module form that would be suitable for industry.
Supported CMS membranes can be synthesized using numerous techniques such as dip coating, spin coating, vapor deposition and sputtering. The ideal structure of such a membrane is shown in FIG. 4. It consists of a thin CMS layer 5 on top of a macroporous, non-selective support 7. The support provides mechanical strength to the membrane, which is a considerable improvement over the hollow fiber configuration. It also has the advantage of being available in various geometries such as flat plates, tubes and disks, which can be used depending on the requirements of the particular application. The support should be an inexpensive material and the pores in the support should be much larger than those in the CMS layer. For example, the pores in the support should be at least twice as large as the pores in the CMS material. In a preferred embodiment of the present invention, the pores in the support are from 5-500,000 times as large as the pores in the CMS material. In the most preferred embodiment of the present invention, the pores in the support are from 10 to 2,000 times as large as the pores in the CMS material.
Although the actual size of the pores in the various support materials can be widely varied, the nominal diameter of the pores in the support material should be greater than 100 xc3x85 (e.g., typical pore sizes in the support material are from 0.1 to 100 xcexcm in diameter). The size of the pores in the CMS material can also vary, but over a much narrower range. For example, the nominal diameter of the pores in the CMS material is generally from 3-100 xc3x85. Preferably, the nominal diameter of the pores in the CMS material is from 3-20 xc3x85. In the most preferred embodiment of the present invention, the nominal diameter of the pores in the CMS material is from 3-10 xc3x85.
CMS membranes have been successfully prepared on porous graphite and ceramic supports. These supports overcome the disadvantage of the hollow fiber configuration by providing durability to the membrane. However, neither of these materials is a good choice for process unit construction compared to metals and alloys. Further, the issue of forming a workable module of the composite membrane needs to be addressed. To successfully use the membrane, it must be put into a module that creates two zones for gas flow separated by the membrane. The critical parts of the module are the points of contact between the membrane and the module wall. These contact points are called end fittings or edge fittings in the case of a planar membrane. The fittings (seals) must provide complete isolation of the two sides of the membrane and should be devoid of any leaks that can create transport through a route other than the CMS layer. It is nearly impossible to fabricate leak free end fittings and modules for graphite and ceramic supported membranes. In the event that modules have been constructed, special end fittings were required, which would increase the cost if the process were commercialized. Thus, graphite and ceramic supports, while a definite improvement over hollow fiber membranes, are not able to meet the requirements of an industrial scale separation process.
One of the first attempts at making supported CMS membranes was by Bird and Trimm (see Bird, A. J. and D. L. Trimm, Carbon Molecular Sieves Used in Gas Separation Membranes; Carbon, 1983; 21; p.177). They pyrolyzed polyfurfuryl alcohol (PFA) on various support materials including silica frits, sintered bronze and copper and iron gauzes. Experiments were carried out to measure the diffusivities of various gases as a function of temperature. The researchers encountered the problem of being unable to create a uniform, defect free layer on any support surface, with the exception of silica frits. The control of the CMS microstructure was also very poorxe2x80x94membranes synthesized under similar conditions exhibited widely varying behavior in terms of gas diffusivities. However, there was some degree of separation obtained between gases, and this was attributed to flow through cracks as well as surface diffusion on the carbon. There was some evidence of activated diffusion as well, and activation energies were obtained for different gas-support material pairs.
Rao and Sircar (see Rao, M. B. and S. Sircar, Nanoporous Carbon Membranes for Separation of Gas Mixtures by Surface Selective Flow; Journal of Membrane Science, 1993; 85; pp. 253-264) developed the xe2x80x9cSurface Selective Flowxe2x80x9d (SSF(trademark)) membrane, in which the primary mechanism for gas separation was the difference in surface flow of various gases on carbon. The membranes were synthesized by coating a layer of poly(vinylidene chloride)-acrylate terpolymer latex on a macroporous graphite disk with a pore size of 0.7 xcexcm. The samples were pyrolyzed at 1000xc2x0 C. in a nitrogen stream, and the coating procedure was repeated to increase the carbon layer thickness. SEM analysis revealed a crack-free membrane with a layer thickness of approximately 2.5 xcexcm. As compared to other separation mechanisms like Knudsen and molecular sieving, surface flow by selective adsorption was found to have several advantages. Components present in low concentrations could be separated, which eliminated the need for a large pressure drop across the membrane. Also, since surface adsorption increased at lower temperatures, ambient operating conditions improved the selectivity. The membrane was used to separate hydrocarbons from hydrogen and hydrocarbon mixtures and provided high selectivities for the former. Graphite supports were also used by Chen and Yang (see Chen, Y. D. and R. T. Yang, Preparation of Carbon Molecular Sieve Membrane and Diffusion of Binary Mixtures in the Membrane; Industrial and Engineering Chemistry Research, 1994; 33; pp. 3146-3153) to synthesize membranes from polyfurfuryl alcohol (PFA). Again, the carbon layer was found to be crack free and its thickness was 15 xcexcm. Diffusivities of gases in the membrane were found to be concentration dependent. The experimental data was explained quite well by the binary diffusivity theory developed by the authors.
Membrane reactors have generated a great deal of interest for use in the selective catalytic reaction of industrial chemicals. There have been several reviews that point to the potential advantages of these novel reactor designs in the form of operational savings that can be realized by increasing conversion efficiencies and product selectivity. Armor, J. N. (1989). Catalysis With Permselective Inorganic Membranes. Applied Catalysis 49(1): 1-25; Armor, J. N. (1992). Challenges in Membrane Catalysis, Chemtech 22(9): 557-563; Armor, J. N. (1998). Applications of catalytic inorganic membrane reactors to refinery products, Journal of Membrane Science 147(2): 217-233; Saracco, G., H. Neomagus, et al. (1999). High-temperature membrane reactors: potential and problems, Chemical Engineering Science 54(13-14): 1997-2017; Saracco, G. and V. Specchia (1994). Catalytic Inorganic-Membrane Reactorsxe2x80x94Present Experience and Future Opportunities, Catalysis Reviews-Science and Engineering 36(2): 305-384; Saracco, G., G. F. Versteeg, et al. (1994) Current Hurdles to the Success of High-Temperature Membrane Reactors, Journal of Membrane Science 95(2): 105-123. Among other advantages, membrane reactors have the potential to exceed equilibrium conversions by selectively removing species from the reaction zone, see Raich, B. A. and H. C. Foley (1995), Supra-Equilibrium Conversion in Palladium Membrane Reactorsxe2x80x94Kinetic Sensitivity and Time-Dependence, Applied Catalysis a-General 129(2): 167-188, or by eliminating undesired reaction pathways in certain reaction networks, see Harold, M. P., V. T. Zaspalis, et al. (1993) Intermediate Product Yield Enhancement With a Catalytic Inorganic Membrane .1. Analytical Model For the Case of Isothermal and Differential Operation, Chemical Engineering Science 48(15): 2705-2725.
Armor points towards the development of highly selective nanoporous membranes for use as membrane reactors observing that membranes with larger pores and lower selectivities require additional downstream separation to recover the product of value. To this end, there have been a number of recent studies exploiting the molecular sieving properties of nanoporous membranes for use in catalytic reactors. See Lafyatis et al. Alfonso and co-workers studied the effects of various feed configurations for propane oxidative dehydrogenation over a V/Al2O3 catalytic membrane/zeolite film composite among other types of hybrid membranes. See Alfonso, M. J., A. Julbe, et al. (1999), Oxidative dehydrogenation of propane on V/Al2O3 catalytic membranes. Effect of the type of membrane and reactant feed configuration, Chemical Engineering Science 54: 1265-272. However, in this case, the intrinsic activity of the zeolite membrane itself suppressed the catalytic selectivity by increasing the overall conversion. Using a supported, silicite-1 membrane, van de Graaf and co-workers were able to demonstrate supra-equilibrium conversion in the metathesis of propene. See van de Graaf, J. M., M. Zwiep, et al. (1999), Application of a silicalite-1 membrane reactor in metathesis reactions, Applied Catalysis a-General 178(2): 225-241; and van de Graaf, J. M., M. Zwiep, et al. (1999), Application of a zeolite membrane reactor in the metathesis of propene, Chemical Engineering Science 54(10): 1441-1445. The membrane selectively transported trans-2-butene over the other species in the reaction zone thereby augmenting the thermodynamic conversion of the reaction from 34% to 38.4%.
Nanoporous carbon (NPC) is a promising material for use as a catalytic membrane in that it is chemically inert under most reaction conditions and thermally stable at temperatures well above 200xc2x0 C. where most industrially relevant reactions occur. See e.g., Foley, H. C. (1995) Carbogenic Molecular-Sievesxe2x80x94Synthesis, Properties and Applications, Microporous Materials 4(6): 407-433; and Kane, M. S., J. F. Goellner, et al. (1996). Symmetry breaking in nanostructure development of carbogenic molecular sieves: Effects of morphological pattern formation on oxygen and nitrogen transport, Chemistry of Materials 8(8): 2159-2171. Formed from the pyrolysis of non-carbonizing natural synthetic polymeric precursors, NPC is a disordered material having a porosity approaching molecular dimensions and has been shown to posses highly shape selective transport properties. Poly-furfuryl (PFA) derived nanoporous carbons have a mode in the pore size distribution of about 0.5 nm as measured from N2 and methyl chloride adsorption isotherms. See Mariwala, R. K. and H. C. Foley (1994), Calculation of Micropore Sizes in Carbogenic Materials From the Methyl-Chloride Adsorption-Itsotherm, Industrial and Engineering Chemistry Research 33(10): 2314-2321. Attempts to use this material in the synthesis of defect free, micron scale film on a structurally stable macroporous support have been highly successful. Membranes have been fabricated using NPC which are selective for the separation of small molecular species. See Acharya, M. and H. C. Foley (1999), Spray-coating of nanoporous carbon membranes for air separation, Journal of Membrane Science 161: 1-5; and Acharya, M., B. A. Raich, et al. (1997), Metal-supported carbogenic molecular sieve membranes: Synthesis and applications, Industrial and Engineering Chemistry Research 36(8): 2924-2930. To the inventor""s knowledge, there have not been any studies appearing in the literature using nanoporous carbon in a catalytic membrane reactor to date.
The selective hydrogenation of olefins has been used by several researchers to characterize the shape selective behavior of metal on nanoporous carbon catalysts. Trimm and Cooper polymerized furfuryl alcohol in the presence of H2PtCl6 and subsequently pyrolyzed the resulting mixture creating a Pt/NPC catalyst. See Trimm, D. L. and B. J. Cooper (1973), Propylene Hydrogenation over Platinum/Carbon Molecular Sieve Catalysts, Journal of Catalysis 31: 287-292; and Trimm, D. L. and C. B. J. (1970), Preparation of Selective Carbon Molecular Sieve Catalysts, Chemical Communications: 477-478. The nanoporosity of the catalyst support was shown to contribute to the selective hydrogenation of propene and 1-butene over isobutene, 3-methylbutene and 3,3-dimethylbutene. Schmitt and Walker fabricated a similar catalyst and demonstrated shape selectivity between 1-butene and isobutane. See Schmitt, J. L. and P. L. Walker (1971), Carbon Molecular Sieve Supports for Metal Catalystsxe2x80x94I. Preparation of the Systemxe2x80x94Platinum Supported on Polyfurfuryl Alcohol Carbon, Carbon 9: 791-796; and Schmitt, J. L. and P. L. Walker (1971), Carbon Molecular Sieve Supports for Metal Catalystsxe2x80x94II. Selective Hydrogenation of Hydrocarbons Over Platinum Supported on Polyfurfuryl Alcohol Carbon, Carbon 10: 87-92. Lafyatis used the conversion ratio of propylene to isobutylene over various NPC derived metal supported catalysts to characterize the reactant shape selectivity. See Lafyatis, D. S. (1992), The Design and Synthesis of Carbon Molecular Sieve Catalysts for Shape Selective Catalysis, Ph.D. Thesis, Department of Chemical Engineering, University of Delaware: 113-141. These ratios were reported to exceed 14 (propylene/isobutylene) at about 30% conversion of propylene. Miura and co-workers have argued for transition state selectivity using a 0.4 nm pore size Ni on NPC catalyst in the decomposition of methanol by producing only CO and H2. See Miura, K., J. Hayashi, et al. (1993), A shape-selective catalyst utilizing a molecular sieving carbon with sharp pore distribution, Carbon 31(5): 667-674. This result was attributed to a shape selectivity of the carbon/metal interface that hindered the formation of a transition state yielding methane in the product distribution.
The incorporation or dispersion of catalytic materials at a molecular level in nanoporous carbon (NPC) has proven to be especially difficult. Previous researchers have traditionally only investigated aqueous-based sources of inorganic metal salts. Several researchers have created platinum (Pt) on NPC catalysts using chloroplatinic acid (H2CL6Ptxe2x80xa2H2O) by pyrolyzing a suspension containing the acid and hydrophobic polymer. See Trimm, D. L., Cooper, B. J., The Preparation of Carbon Molecular Sieve Catalysts, Chem. Commun. pp. 477-478 (1970); Trimm, D. L., Cooper, B. J., Propylene Hydrogenation over Platinum/Carbon Molecular Sieve Catalysts, J. Catal., V31, pp. 287-292 (1973); Schmitt, J. L., Jr., Walker, P. L., Jr., Carbon Molecular Sieve Supports for Metal Catalystsxe2x80x94I. Preparation of the Systemxe2x80x94Platinum Supported on Polyfurfuryl Alcohol Carbon, Carbon, V9, pp. 791-796 (1971); Schmitt, J. L., Jr., Walker, P. L., Jr., Carbon Molecular Sieve Supports for Metal Catalystsxe2x80x94II. Selective Hydrogenation over Platinum Supported on Polyfurfuryl Alcohol Carbon, Carbon, V10, pp. 87-92 (1972); and Lafyatis, D. S., Mariwala, R. K., Lowenthal, E. E., and Foley, H. C., Design and Synthesis of Carbon Molecular Sieves for Separation and Catalysis. Synthesis of Microporous Materials: Expanded Clays and Other Microporous Solids, M. L. Occelli and H. Robinson, eds., Van Nostrand Rienhold, New York, pp. 318-332 (1992). Aqueous acid-based catalytic precursors inhibit thin film formulation because the polymerization of furfuryl alcohol is known to be acid catalyzed. Hence, the viscosity and other properties of the membrane precursor are unstable. Furthermore, the prior art aqueous-based multiphase system tends to lead to low dispersions of the metal in the catalyst.
To circumvent the disadvantages of the prior art, the present invention embodies a new approach to creating NPC catalytic membranes by using a non-polar organometallic catalytic precursor. The precursor is soluble in both PFA and acetone. This allows for efficient introduction into the spray coating methodology that is known in the art and is described by Acharya, M., Foley, H. C, Spray Coating of Nanoporous Carbon Membranes for Air Separations, J. Membrane Science, V161 pp. 1-5 (1999). The application of the single-phase nanoporous carbon polymer precursor creates a catalytically active NPC membrane with the catalyst being dispersed in the membrane close to the molecular level. This degree of dispersion is a dramatic improvement over the prior art, such as that disclosed by Lafyatis et al. The invention allows for the creation of a metal-on-carbon catalyst that has significantly higher intrinsic activity over prior catalytic membranes due to an increase in the available surface area of the catalyst.
The present invention provides for a synthesis method to disperse catalytic materials in nanoporous carbon membranes and the unique catalytically-active membranes so produced. Nanoporous carbon membranes on metal supports, previously referred to by those skilled in the art as carbogenic molecular sieves (CMS), are disclosed in prior U.S. patent application Ser. No. 08/671,698, (now U.S. Pat. No. 5,972,079) which is incorporated herein by reference. The invention presented uses a non-polar organometallic precursor of a catalyst in solution with a non-polar solvent. A nanoporous carbon polymeric precursor, such as poly-furfuryl alcohol, is added to the solution forming a viscous but single-phase nanoporous carbon (NPC) precursor. The non-polar solvent is flashed off slowly at a heating rate no greater than 5xc2x0 C./min and held at 70xc2x0 C. until the original viscosity of the carbon polymeric precursor is restored. Subsequent pyrolysis of the nanoporous carbon precursor at elevated temperatures produces the catalytic material with excellent (i.e., highly dispersed) distribution of the metal catalyst in the nanoporous carbon membrane.
In addition to the inherent properties of NPC membranes, the catalytically-active NPC membranes described herein have a very high degree of catalytic metal dispersion. Additionally, the membranes have excellent shape-selectivity properties and can be tailored to suit different applications by varying pore sizes.
The present invention is a new application of carbogenic molecular sieve (CMS) membranes which possess catalytic properties, high mechanical strength, simple fabrication procedure and are readily assembled into modules. These composite membranes, which comprise CMS on a porous support material, can be used for small molecule separations or for combined separation and chemical reaction, especially catalytic reactions. In the first application, these membranes are used strictly for separation of molecules, while in the second instance, a catalyst can be incorporated within the module to convert it into a reactor.
The membrane is a composite that has the mechanical strength of the support and molecular sieving properties of the CMS materialxe2x80x94characteristics that are not available in either material separately. This is the major advantage of the invention disclosed herein.